It is known to coat the wear surfaces of wear parts, such a seals, dies, engine parts, and other apparatus with a layer of material harder than the underlying material of the part in order to reduce the wear rate of the part. Diamond film deposited by CVD (chemical vapor deposition), as is described, for example, in "Diamond Coatings" by W. J. P. van Enckevort, Journal of Hard Materials, vol. 1, No. 4, 1990, pp 247-269, is particularly useful for this purpose, since diamond has not only great hardness but also a relatively low coefficient of friction.
Among the most severe applications for wear surfaces are various seals for rotating shafts to seal their passage through a wall. Typically such seals have mating flange members mounted on the shaft and on the wall, respectively. When in service, the flanges rotate against each other under mutual pressure and form a seal between their mating faces under these conditions. Polycrystalline diamond would appear to be an almost ideal material for use on the wear surfaces of rotary seals, since in addition to its above-mentioned favorable wear qualities, it also is quite resistant to most chemical corrosion. However, one problem with diamond in such an application is that if the surface of the diamond coating is not highly even and polished, even very small rough spots on the face can rapidly cause severe damage to the mating face, even if it too is coated with diamond. In most cases it is desirable that the second mating member not be coated with diamond, since such a coating is relatively expensive. Therefore, ceramic materials, such as silicon carbide, silicon nitride, aluminum oxide, or another ceramic or composite materials are often used. These materials are much softer than diamond and are immediately degraded by any roughness or unevenness of the diamond-coated opposing surface.
While the above problems can be dealt with by careful finishing of the diamond coating, such finishing is very expensive because of the hardness of the diamond. Also, in the course of the CVD growth process for the coating, there is a tendency for ever larger faceted crystallites to form as the layer increases in thickness. Individual ones of these faceted crystallites may protrude as asperities from the surface and cause severe damage when sliding against a mating surface. Such asperities are difficult to remove.